Field of the Invention
The present invention relates to a method for manufacturing an optical element and particularly relates to a method for manufacturing an optical element for use in an optical apparatus, such as cameras and videos, by press-molding.
Description of the Related Art
Heretofore, a method for obtaining an optical element by press-molding glass which is heated and softened is known. However, in recent years, a technique of molding a glass material having a special optical constant has been required with an enhancement of performance of a lens unit.
The composition of the glass material of a special optical constant sometimes become a thermally unstable composition. When the glass material of the composition is heated to a molding temperature, the generation amount of volatile constituents increases. When a lens is hot-molded using such a glass material, a foggy lens in which air bubbles are generated due to the generation of volatile constituents is molded, so that the appearance becomes poor. Therefore, when molding the glass material of a special optical constant, a technique of suppressing the generation of volatile constituents is required.
As a former technique of suppressing the generation of volatile constituents, a method for pressurizing the atmosphere in a chamber to be equal to or higher than the vapor pressure is disclosed (Japanese Patent Laid-Open No. 2004-107145).
However, in the former technique described in Japanese Patent Laid-Open No. 2004-107145, the pressure from the atmosphere does not act on the surface where a mold and glass contact each other, and the pressurization force to the glass decreases, so that air bubbles due to volatilization are generated in the molded surface. Furthermore, although the standard of “equal to or higher than the vapor pressure” is described, the material of a special optical constant, particularly a phosphate-based glass containing Bi2O3 in a proportion of 10 mass % or higher and 30 mass % or lower, has had a problem to be solved such that the effect of suppressing volatilization is not obtained even when pressurization exceeding the vapor pressure of metallic elements is performed.